Fix git-archive invocation for RHEL 6

Apparently, on RHEL 6, the valid archive type is tar, not tar.gz.
It causes git-archive and thus makerpm.sh to abort with:
  fatal: Unknown archive format 'tar.gz'

To fix, use a regular pipeline.

Change-Id: I9174ad1d1c0e087f46ecd31eade61eeea75cb9cb
Signed-off-by: Pete Zaitcev <zaitcev@kotori.zaitcev.us>
Reviewed-on: http://review.gluster.org/5271
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>